1. Field of the Invention
The present invention relates to a Ni-plated layer with a biaxial texture, which is excellent in toughness and magnetic properties and useful as substrate for coating YBCO Super conductor cable. Also, the present invention is concerned with an electroplating process of and an apparatus for preparing the Ni-plated layer.
2. Description of the Prior Art
In a polycrystalline material, a texture refers to a single cluster structure consisting of a number of crystal particles which have the same crystal orientation in a polycrystalline material. The texture is generally divided into two groups: fiber texture and three-dimensional texture.
In order to better understand the background of the invention, a description will be given of the texture in conjunction with FIG. 1.
FIG. 1 schematically shows textures in board planks. FIG. 1a is a schematic view showing the absence of textures. Shown in FIG. 1b is a uni-axial texture, generically designated xe2x80x9cfiber texturexe2x80x9d. The fiber texture is usually found in the columnar crystals of cast materials, vapor-deposited metal films, electroplated layers, extruded materials, and drawn wires. In the fiber texture, as seen in FIG. 1b, a uniform crystal orientation  less than hkl greater than  appears in the c-axial direction of the material while crystal orientations are randomly arranged on the plane composed of the a-axis and the b-axis.
In contrast, the biaxial texture, which is exemplified in FIG. 1c, has the crystal orientation directed uniformly to the c-axis of the material as well as makes the crystal grains arranged uniformly in the direction of the a-axis and b-axis. Thus, the biaxial texture is composed of three-dimensionally uniform crystal orientations and crystal planes just like single crystals. This texture can be seen in rolled board planks.
A metal board of a biaxial texture exhibits characteristic physical properties. For instance, it is well known that, a biaxial texture is high in mechanical toughness because a small difference in direction angle between grain boundaries allows the biaxial texture to be of low interfacial energy. In addition, as in an Fe-6.5% Si alloy, the biaxial texture assures better characteristics in magnetic substances, ferroelectrics, and high critical temperature superconductors. Particularly, on the basis of the phenomenon that a coating layer tends to conform itself to the orientation of the substrate, a substance with a single crystal structure or a biaxial texture is used, for the most part, as a coating matrix so as to provide the coating layer with such a biaxial texture especially in the case that ferroelectrics of a perovskite structure, or YBCO high critical temperature superconductors are prepared by a thin film coating process. For instance, ORNL of the USA developed a so-called RABiTS process, which was a turning point in YBCO superconductor tape preparation. Taking advantage of the cubic crystal texturization phenomenon that the [001] axis is oriented vertically to the surface upon the recrystallization of cold-rolled FCC metal, the RABiTS process allows board planks with biaxial textures to be used as substrates in a subsequent thin film process. That is, after a rolled Ni board with a biaxial texture is prepared by rolling, a buffer layer and a YBCO superconducting thin film are coated on the Ni board by a vacuum deposition method. In this case, the coating grows into a biaxial texture as a result of conforming itself to the crystal orientation of the substrate having a biaxial texture. Removal of high angle crystal boundaries from the plane composed of the a-axis and the b-axis brings about a noticeable increase in the critical current density of superconduction. Therefore, a substrate with a biaxial texture has a technically very important significance.
A coating layer formed under a particular condition by electroplating is of a uniaxial texture because it shows a vertical orientation in the direction of the c-axis perpendicular to the matrix plane, but random orientations on the plane formed by the a-axis and the b-axis, as exemplified in FIG. 1b. Thus, it has been regarded as impossible to prepare a plated layer of biaxial texture in which crystal orientation is arranged uniformed in the direction of the c-axis as well as the a-axis and the b-axis. No research has been conducted on the preparation of plated layers which are of biaxial texture. If a biaxial texture had been realized by electroplating, it should have been possible to continuously produce tapes or boards of biaxial texture at lower costs with significantly greater ease compared with the rolling process which requires many rolling steps and intermediate thermal treatments.
There were some study cases that apply magnetic fields for plating processes. Most of them are directed to plating efficiency or deposition rates and few pertain to the orientation of plated layers.
J. McDonald found that micro-stresses became anisotropic within a coating which was deposited under a magnetic field, asserting that nickel coatings and iron-nickel alloy coatings, when being deposited at the lines of magnetic force parallel to the electrode, show anisotropy of micro stress and the orientation is dependent on the deposition condition of the magnetic field and the alloy composition. This resulted from the study on the uni-axial texture exhibiting vertical orientation, but was not extended to biaxial textures.
M. Perakh made an observation of that, under a magnetic field, iron, nickel and cobalt coatings were not affected in crystal orientation, but their surfaces abnormally grew in the direction of the lines of magnetic force to a rough state.
A. Chiba reported that the crystal orientation of a nickel coating changed depending on the magnetic field applied and became random at a magnetic field intensity of 0.6T in a watts bath. However, this is also confined to uni-axial textures.
The discordance among the researchers"" results is, to the inventors"" knowledge, attributed to the fact that their research was conducted under such a plating condition that account could not taken of only pure magnetic field effect or the intensity of magnetic field was too weak to provide magnetic field effects.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a Ni-electroplated layer which is of a biaxial texture such that it is superior in toughness and magnetic properties.
It is another object of the present invention to provide a process of preparing Ni layers of biaxial texture at a low cost by electroplating.
In accordance with an embodiment of the present invention, there is provided a process of preparing an Ni-plated layer, comprising the steps of: forming an Ni-plated layer of biaxial texture under a magnetic field by electroplating; and subjecting the Ni-plated layer to thermal treatment to enhance the biaxial texture.
In accordance with another embodiment of the present invention, there is provided an electroplating bath, capable of allowing the formation of a Ni-plated layer of biaxial texture, which comprises an anode and a cathode arranged therein and an electromagnet installed at its external side.
In accordance with a further embodiment of the present invention, there is provided an Ni-plated layer of biaxial texture, formed on a polycrystalline matrix, wherein peaks measured on a xcex8-rocking curve have a FWHM(Full Width at Half-Maximum) of 7xc2x0 or less in terms of the misorientation on the c-axis and peaks measured on T-scan have a FWHM of 21xc2x0 or less in terms of the misorientation on the plane formed by the a-axis and the b-axis.